Mission Viejo / / is a city located in southern Orange County, California, U.S. in the Saddleback Valley. Mission Viejo is considered one of the largest master-planned communities ever built under a single project in the United States, and is rivaled only by Highlands Ranch, Colorado, in its size. The city has a 2011 estimated population of 93,483. 
Mission Viejo is suburban in nature and culture. The city is mainly residential, although there are a number of offices and businesses within its city limits. The city is known for its picturesque tree-lined neighborhoods, receiving recognition from the National Arbor Day Foundation. The city's name is a reference to Rancho Mission Viejo, a large Spanish land grant from which the community was founded.
Mission Viejo was named the safest city in the United States by the Morgan Quitno crime statistic survey (compiled from FBI data). The city was also ranked safest in California and third in the nation by CQ Press.
Mission Viejo was purchased by John Forster, a Mexican also known as Don Juan. During the Mexican-American War, Forster provided fresh horses to United States military forces which were used on the march of San Diego to retake Los Angeles.
Mission Viejo was a hilly region primarily used as cattle and sheep grazing land, since it was of little use to farmers. This city was one of the last regions of Orange County to be urbanized due to its geologic complexity. In 1960, early developers dismissed most of the land in Mission Viejo as simply "undevelopable". 
Donald Bren, an urban planner who later became the president of the Irvine Company, drafted a master plan which placed roads in the valleys and houses on the hills, and contoured to the geography of the area. The plan worked, and by 1980 much of the city of Mission Viejo was completed. During the late 1970s and the 1980s, houses in Mission Viejo were in such high demand that housing tracts often sold out before construction even began on them. The houses and shopping centers in the city are almost uniformly designed in a Spanish mission style, with "adobe"-like stucco walls and barrel-tile roofs. Many point to Mission Viejo as the first and largest manifestation of Bren's obsession with Spanish architecture. Bren's company was also the creator of the developments in Irvine, and Newport Beach suburbs. The company expanded its operations and went on to build the Lakes project in Tempe Arizona, Mission Viejo Aurora in Colorado and was the initial master planner of Highlands Ranch, both in the Denver Metropolitan area.
The seal of the city of Mission Viejo was designed and drawn by Carl Glassford, an artist and former resident of the city.